Everything that has been observed, regarding overflows, truncations, ELSE clauses executing and so on...is all documented in Books Online, under the following topics:

SET ANSI_WARNINGS

SET ARITHABORT

SET ARITHIGNORE

The various behaviours described in this thread can all be varied, and explained, by reading those references.

The other thing is that a scalar sub-query like in the example (SELECT a FROM @tab) always returns a value - since it returns a column reference. If no rows are produced, the scalar sub-query presents a NULL.